Approximately 170 million people are infected by hepatitis C virus (HCV) worldwide and many of these will go on to develop disease, primarily cirrhosis of the liver and hepatoma. A vaccine is required to help limit spread of the disease. Most effective vaccines developed to date elicit neutralizing antibodies (NAbs). There have been at least two major roadblocks en route to developing a component that elicits NAbs to HCV. The first is the lack of a conventional neutralization assay because of an inability to culture HCV in vitro. This roadblock has recently been at least partly dismantled by the development of systems for culturing HCV, albeit currently with certain limitations. The second roadblock is the great sequence variation of HCV found in infected donors that suggests a vaccine should elicit NAbs effective against a wide variety of different viruses, i.e. the vaccine should elicit broadly neutralizing antibodies. We propose a program to investigate the necessary conditions to induce a strong neutralizing antibody response against multiple HCV isolates and use the knowledge gained to design a NAb-inducing component of an HCV vaccine. We will systematically dissect NAb responses to HCV in natural infection, and isolate mAbs that are most potent and broad against neutralizing diverse isolates of HCV (Aim 1). These prototype mAbs will help us to identify neutralizing epitopes on HCV and we will explore the interaction between these epitopes and NAbs at the molecular level (Aim 2). We will apply knowledge gained from these molecular studies to the design of immunogens in order to produce NAb responses in animals (Aim 3). The prototype broadly neutralizing mAbs and Abs from animals immunized with the novel immunogens will be tested in a small animal model (Aim 4).